174 



INTRODUCTION 



C. Flies or Two-winged Insects (Diptera). 



In Hermann Miiller's opinion (' Fertilisation/ p. 36) most species of flies probably 

 visit flowers, but this only applies to the first main division of the order, the Diptera 

 brachycera, which are of very great importance in relation to pollination, while 

 the species of the second great division, the Diptera nematocera, are almost useless 

 in this respect, only some of the Tipulidae being common visitors of flowers, while 

 a few tiny moth-flies (Psychodidae) pollinate Aristolochia, Arum, Adoxa, and 

 Chrysosplenium. It will therefore suffice to describe the organs employed in pro- 

 curing nourishment from flowers, and the way they are used in those flies which 

 are of special importance for the transference of pollen. I think that this cannot 

 be done better than by following the admirable account given by Hermann 

 Muller (' Fertilisation,' pp. 36 et seq.). 



a#*"'*'*'*'i'-, 



Fig. 72. Mouth-parts of Bristalis ( x 7) (after Hermann Muller). (1) Head of E. arbustorum, with 

 retracted proboscis; from the side. (2) Ditto, from below. (3) Ditto, with extended proboscis; from 

 above. (4) Extended proboscis of E. tenax ; from the side. (5) Ditto, from below. (6) Ditto, from above, 

 a, eye; bb^ antennae; c c, end-flaps of proboscis ; d c\ their inferior segments; d, groove on the upper 

 side of the tip of the proboscis ; yj contractile middle part of the proboscis ; g^ contractile base of 

 proboscis ; A, upper lip (labrum), grooved below to receive the unpaired piece (z"), which probably 

 represents the two fused mandibles; k, maxilla; /, maxillary palp; in tn, edges of the cavity on the 

 under-side of the head into which the entire proboscis is withdrawn ; w, occipital foramen. 



The family of Hover-flies (Syrphidae) includes those well-known insects which 

 hover as if fixed to a point in the air, then rapidly dart to one side and act as 

 before. They contribute far more to the pollination of our native flowers than 

 all the other Diptera put together, for most of their numerous and often very common 

 species mainly or exclusively depend on flower-food, and in connection with this 

 they exhibit very special adaptations for securing pollen and nectar alternately. 

 To illustrate the mouth-parts of flies, and their relation to flowers, H. Muller 

 therefore selected Eristalis and Rhingia, two hover-flies which are highly developed 

 in this respect, and at the same time very common. 



In Eristalis the completely extended proboscis is clearly seen to be composed 

 of three successive segments (Fig. 72, 4, 5, 6, and Fig. 73, i). (i) The membranous 

 basal piece (^), which bears at its front end two unpaired {h, i) and two paired {k, k) 



